Fate of Diverted Mississippi River Nitrate in a Vegetated Brackish Coastal Marsh.

The Caernarvon diversion meters Mississippi River water into the brackish and salt marshes of Breton Sound during spring flooding.  Elevated levels of nitrate delivered through the Caernarvon diversion have recently sparked concerns that nitrate loading is affecting plant resilience, in particular, potentially affecting belowground biomass. This concern resulted from observation that the fresh and brackish Breton Sound marshes suffered extensive damage from Hurricane Katrina. Our hypothesis is that the nitrate levels in the Mississippi River are not high enough to affect belowground biomass and that most nitrate is removed by denitrification. To test this hypothesis, 12 plant-sediment cores were collected from a brackish marsh located proximal to Delacroix, Louisiana. Six received dionized water (control), while another six (treatment) received 2 mg/L of ¹⁵N-labeled potassium nitrate twice a week for three months. A set of three control and treatment cores were destructively sampled after three months and analyzed for ¹⁵N in the aboveground and belowground biomass as well as denitrifying enzyme activity.  The remaining three treatment cores received 10 mg/L of ¹⁵N-labeled potassium nitrate twice a week for one month and the fate of the ¹⁵N- nitrate was used to distinguish different removal pathways, including denitrification, surface algae uptake, incorporation into aboveground biomass, and incorporation into belowground biomass. Preliminary results show that just 24 hours after the addition of 2 mg/L nitrate, levels were below detection, which suggests denitrification or surface algae are the major removal pathways. Results of the partitioning of ¹⁵N by soil microbes, surface algae, aboveground biomass, and belowground biomass will be presented.